Attenuators are used in applications that require signal level control. For microwave applications, absorptive attenuators, i.e., attenuators which absorb some of the signal in the attenuator itself, are preferred over reflective attenuators which reflect a portion of the input signal back to its source. The important parameters of an absorptive attenuator are its accuracy as a function of frequency, its return loss and its stability over time and temperature. It is known that variations in temperature can affect various component parts of a microwave system causing differences in signal strengths at different temperatures. Much time, effort and expense has gone into the components of such systems in an effort to stabilize them over various temperature ranges. This greatly increased the cost of microwave systems that must be exposed to wide temperature ranges.
A system which has been developed to simply and easily overcome temperature variation problems in a microwave attenuator is the temperature variable attenuator shown and described in U.S. Pat. No. 5,332,981 to Joseph B. Mazzochette et al., issued Jul. 26, 1994, entitled "Temperature Variable Attenuator", which is incorporated herein by reference. This device comprises at least two temperature variable resistors. One of the resistors has a temperature coefficient of resistance which is different from that of the other resistor. Preferably, one of the resistors has a positive temperature coefficient of resistance and the other resistor has a negative temperature coefficient of resistance. The temperature coefficient of resistance of the two resistors are such that the attenuation of the attenuator changes at a controlled rate with changes in ambient temperature, but wherein the impedance of the attenuator remains constant at the attenuation changes. Although this device operates satisfactorily, it is often desirable to extend the use of the device by making it a voltage variable attenuator and to provide better compensation at high temperatures.